fix: fix include_z when setting stage positions value in PositionTable widget

[tlambert03]

fixes #380

for now, this will set the "include z" checkbox to True if any of the positions have a Z position. If only some of them have a z position, it will emit a warning.

If we need the feature in the future, we could further elaborate to somehow let some rows have a Z position of None while others don't

[tlambert03]

tests are failing, but i think it might be the assertion in the test itself that is no longer correct

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.72%. Comparing base (d58003b) to head (30371b3).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #381   +/-   ##
=======================================
  Coverage   90.71%   90.72%           
=======================================
  Files          85       85           
  Lines        9188     9197    +9     
=======================================
+ Hits         8335     8344    +9     
  Misses        853      853           

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